The residues of organic material produced during combustions are referred to as ash. The composition of ash particles greatly depends on the combustible material and extends from residual carbon and minerals (quartz) over metallic compounds (mostly oxides and (bi-) carbonates, such as Al2O3, CaO, Fe2O3, MgO, MnO, P2O5, K2O, SiO2, Na2CO3, NaHCO3) up to toxic substances, such as heavy metals (e.g. arsenic and zinc) and dioxins. The residual carbon contained in the ash with an amount of up to 20 wt-% for the most part is centered in a coal core, which is surrounded by metal oxide compounds. Such ashes are not suitable for simple dumping, but must be declared as hazardous waste, since they are hygroscopic and with improper dumping the soluble components leached out thus can get into the groundwater. This form of ash is not usable either as filler in the cement industry, because the residual carbon content deteriorates the binding properties.
The problem of the residual carbon content in ash, in particular that of the fly ash obtained in coal-fired power plants with a bulk density of 0.9 to 1.1 kg/l, is known just as experiments to penetrate the metal oxides protectively enveloping the carbon core with microwave radiation. EP 0 521 059 B1 for example describes a method for determining the carbon content in fly ash. For this purpose, an ash sample is collected in a measuring chamber which acts as microwave resonance space, and the sample is exposed to a microwave radiation. By measurements of the absorption of sample and reference materials, the dielectric constant of the sample material can be determined and thus ultimately be used to estimate the residual carbon content.
U.S. Pat. No. 4,705,409 likewise describes the use of microwave radiation in conjunction with the determination of the carbon content in fly ash. For this purpose, the fly ash is exposed to a microwave radiation for absorbance measurement. Part of microwave radiation is absorbed hereby by carbon, the rest of the radiation is absorbed in an absorption fluid. From the difference of the temperature of this absorption fluid in measurements with and without ash sample the carbon content can be inferred. Due to the absorption of the microwave radiation, a partial reduction of the carbon content in the fly ash can occur, which requires, however, a comparatively high radiation energy.
A process for the thermal treatment of granular solids, in particular for roasting ores, in a reactor with fluidized bed in which microwave radiation from a microwave source is fed into the reactor, is known for example from DE 102 60 745 A1. To improve the utilization of energy and the feeding of the microwave radiation, a gas or gas mixture is introduced into a mixing chamber of the reactor from below through a central supply tube, wherein the gas supply tube is at least partly surrounded by a stationary annular fluidized bed fluidized by supplying fluidizing gas. The feeding of the microwaves is effected through the central gas supply tube.